As the border conflict between Thailand and Cambodia intensifies, accusations regarding Thailand’s use of cluster munitions have raised both legal and humanitarian concerns. Although Thailand is not a signatory to the 2008 Convention on Cluster Munitions (“CCM”), which prohibits parties from using, producing, or stockpiling cluster munitions, its actions may still violate broader principles of international humanitarian law (“IHL”).
On 25 July 2025, Cambodia’s Ministry of National Defense alleged that Thai forces deployed heavy weapons and cluster munitions at least seven locations along the border, including areas near civilian settlements. These cluster munitions were inherently indiscriminate, scattering submunitions over wide areas and often leaving behind unexploded submunitions that posed long-term risks to civilians. Cambodia has now condemned the strikes as a violation of international law, warning that such attacks could constitute war crimes.
The Thai Armed Forces Headquarters initially denied these allegations. However, later that same day, a Royal Thai Army press release and a military spokesperson admitted the use of cluster munitions, stating they were deployed “when necessary against military targets” and “to enhance target destruction capabilities” while adhering to the “principle of proportionality”. Thailand also defended its position by noting that it is not a signatory to the CCM and thus is not subject to such obligations. The argument presented by Thailand distorts the reality of international law, which acknowledges other sources of international law and obligations beyond written treaties and conventions.
While this is true that a state is not bound by a convention it has not ratified, Thailand is therefore not bound by the CCM. However, such an interpretation relies on a narrow reading of international law and ignores the universal applicability of customary IHL—rules derived from “a general practice accepted as law”, which bind all parties to a conflict, including Thailand.
In this respect, having not ratified the CCM treaty does not absolve Thailand from international obligations under the IHL. As a state party to the 1949 Geneva Conventions, Thailand is obligated to uphold the principles of distinction, proportionality, and precaution during armed conflict. It must avoid an attack of a nature that fails to distinguish between military objectives and civilians or civilian objects without discrimination. Any breach of these obligations could constitute a grave violation of the 1949 Geneva Conventions and may be prosecuted as a war crime under the Rome Statute of the International Criminal Court (“Rome Statute”). In particular, the fact that cluster munition was used on Cambodian territory and injured Cambodian civilians, a state party to the Rome Statute, triggered the territorial and personal jurisdiction of the International Criminal Court. Given that the prohibition of cluster munitions is universal and form part of customary IHL, the use of such munition likely amounted to war crime of “attacking civilians and/or civilian objects” under articles 8(2)(b)(i)-(ii) or war crime of “excessive incidental death, injury, or damage” under article 8(2)(b)(iv) of the Rome Statue. This assessment would, however, depend on the availability of sufficient evidence to assess: (i) the exact nature, scale, and effect of the cluster munitions deployed; (ii) the known area of ‘spread’ of sub-munitions; (iii) the characteristics of the area targeted and the proximity of civilian residential zones to legitimate military objectives; or (iv) the availability of alternative means of attack.
In light of the foregoing, cluster munitions pose high risks to civilians and are particularly difficult to use lawfully in populated or culturally significant areas, such as those near Cambodia’s UNESCO-listed Preah Vihear Temple. Their widespread effects and high failure rate make it difficult to comply with these IHL principles. With that being said, any use of cluster munitions that cause indiscriminate or disproportionate harm to civilians may still constitute a violation of IHL, even for states outside the CCM.
It is worth noting that this is not the first time Thailand has faced scrutiny over the use of cluster munitions. In 2011, similar munitions were reportedly deployed during previous clashes with Cambodia during the border conflict over Preah Vihear Temple.
This drew international pressure for Thailand to join the CCM. Another notable precedent can be seen in Yemen, where Saudi Arabia—a non-signatory to the CCM—used cluster munitions in its military campaign starting in 2015. The resulting humanitarian toll led to investigations by the United Nations and Amnesty International, which documented civilian casualties and the long-term dangers of unexploded submunition. These reports have led to an armed restraint, whereby the United States suspended its cluster munition transfers to Saudi Arabia in May 2016.
Similarly, Thailand’s current position reflects a narrow legal interpretation. Even without CCM membership, its military actions must still comply with the fundamental principle of IHL. As global consensus shifts toward banning cluster munitions, Thailand risks not only reputational damage but also diplomatic fallout. Ultimately, international law is not only about what is signed, but it is also about what is universally expected. In today’s interconnected legal framework, signing a treaty is not the only path to accountability. The law of armed conflict, particularly those protecting civilians, is binding on all, regardless of the paperwork.
Author: Vireakboth Im